When sensory hair cells in the ears of humans and other mammals are killed by loud sounds, drug toxicity, infections, or other causes, they are not effectively replaced. As a result, millions of people have permanent, acquired hearing and balance deficits. That is not the case in non-mammals, like fish, amphibians, and birds. In their ears, hair cell loss triggers the regenerative replacement of lost hair cells and that is soon followed by the restoration of hearing and balance. The new hair cells arise from supporting cells within the same epithelia. This application seeks support for a project that began in 1982, which will test the hypothesis that the unique intercellular junctions and massively thickened actin bands in postnatal mammalian supporting cells play a role in limiting the regeneration of hair cells in mammals. Intercellular junctions have recently been identified as signaling sites tha play crucial roles in the regulation of organ growth and in regenerative turnover that occurs in many epithelia. The proposed investigations also will seek to define the supramolecular organization and protein composition of the actin bands that grow to occupy nearly 90% of the area inside the junctions average supporting cells in the vestibular epithelia of adult humans and rodents. Rodent supporting cells lose regenerative plasticity in a manner that is almost perfectly correlated with the postnatal thickening of these junctional actin bands. Thickening does not occur in the supporting cells of non-mammals that regenerate hair cells. The central hypothesis will be tested in vitro and in vivo using mouse genetics and pharmacological inhibitors. Outcome from the proposed tests and the answers to other questions addressed in the proposed investigations will identify specific proteins which hold the potential to become targets for therapeutic approaches aimed at translating laboratory discoveries into treatments for millions of patients who are affected by permanent, highly forms of hearing loss and balance dysfunctions that contribute to communication disorders as well as to instability and falls in the elderly.